The good news is fisheries managers, business owners and politicians are all aware of the importance of walleyes and resources are being applied to learn about the leading causes of the decline and devise strategies to potentially help the species recover.

Such science doesn't happen overnight, of course, and several steps will be necessary before such programs can be tested, much less implemented.

The authors were Andrew Rypel, formerly of the Wisconsin Department of Natural Resources and currently at University of California, Davis; Daisuke Goto and Jake Vander Zanden of the Center for Limnology at the University of Wisconsin; and Greg Sass of the Wisconsin Department of Natural Resources.

The study was funded by the Wisconsin Department of Natural Resources, the United States Geological Survey and the National Science Foundation’s Long Term Ecological Research Program in North Temperate Lakes.

The researchers analyzed production statistics collected between 1990 and 2012 for adult walleye populations in 473 northern Wisconsin lakes.

The data showed annual walleye production across all the study lakes decreased by 27% over the 22 years.

It now takes 1.5 times longer to produce the same amount of walleye biomass, or fish weight, as it did in 1990, according to the researchers.

Some lakes fared worse than others, however. Lakes that showed good levels of natural reproduction sustained good levels of production.

Lakes with a mixture of both stocked and naturally reproducing walleye experienced an average decline of 47%, while lakes with only stocking and no natural reproduction declined by 63%.

Fisheries scientist Andrew Rypel holds a walleye on a northern Wisconsin lake. Rypel formerly worked as an ecologist at University of Wisconsin and the Wisconsin Department of Natural Resources and is currently an associate professor and the Peter B. Moyle and California Trout Chair in Coldwater Fish Ecology at the University of California, Davis.

Photo contributed by Andrew Rypel.

He learned to love fishing from his grandfather and father, including on Pewaukee Lake, the Fox River, Lake Michigan and at a family cabin in northern Wisconsin.

"So, fishing the vast supply of Wisconsin lakes and streams was an essential, maybe even the central, aspect to my upbringing," Rypel said.

Rypel earned an undergraduate degree in environmental science from Saint Louis University, a masters in fisheries from Auburn University and a PhD in Ecology from the University of Alabama.

From 2012-'17 he was employed at the Wisconsin Department of Natural Resources in the Science Services (now Office of Applied Science) department.

His work focused on panfish research, lakes classification and walleye research.

Rypel left the DNR last year to accept a position as associate professor and the Peter B. Moyle and California Trout Endowed Chair in Coldwater Fish Ecology at the University of California, Davis.

The recent paper was published with data garnered in part from tribal spearing harvests.

"Most people don’t realize, but Wisconsin has probably the best datasets on walleye in the world," Rypel said, referring to the management system put in place after the 1980s court cases that affirmed Chippewa off-reservation fishing and hunting rights.

Each walleye harvested by the tribes is measured and sexed. The data has produced high quality information on population estimates, growth information, size-structure data and creel censuses of harvest, Rypel said.

He credits the architects of the system, including DNR fisheries employees Michael Hansen, Steve Hewett and Mike Staggs, with the "scads" of data available now to researchers.

Rypel said many of the questions now surrounding walleye biology and management are fundamentally science questions.

For example: Are we really seeing declines in walleye populations? If so, why might populations be struggling? What can managers do about it? Does stocking work? What might such patterns mean for sustainable harvest of walleye?

The first question and part of the second have been answered.

Rypel said walleye populations in Wisconsin that are retaining natural reproduction are still doing great and have high production rates. For example, Escanaba Lake in Vilas County was, over time, the most productive walleye fishery in the state and still has excellent year-over-year natural recruitment of walleye.

The issue is with populations that have lost or are losing natural reproduction. In these populations, there winds up being a mix of natural reproduction and stocking, or alternatively, no natural reproduction and only stocking.

The decline in these lakes is not necessarily because the lakes are being stocked. Rather, the data suggest that the underlying habitat that used to help produce all those walleye is changing.

The fact that walleye production declined so strongly in “stocking-only” lakes seems particularly revealing, Rypel said.

These lakes, which were likely marginal walleye lakes to begin with, seem to be getting even worse for the species.

"But perhaps the most eye-opening part of the story is that the number of lakes classified as natural reproduction lakes is getting smaller while the number of combination lakes (stocking and natural reproduction) is increasing," Rypel said, "So in a nutshell, many walleye populations seem to be headed in the wrong direction."

What's causing the declines? Studies by Gretchen Hansen, formerly of the Wisconsin DNR and now with the Minnesota DNR, have pointed to warming water as being harmful to walleyes and beneficial to largemouth bass.

Overharvest and habitat degradation have also likely contributed to the walleye decline, Rypel said.

Such changes are most often complex and involve multiple factors, Rypel said.

The big question – what can be done to help walleye populations – remains to be answered.

The Wisconsin Walleye Initiative, which stocks large fingerlings, is now in its fifth year and reviews of its effectiveness will soon be made.

The walleye production problem is not likely to be solved quickly. Rypel said the large scale erosion in the productivity of walleye populations has not been seen before.

But he also has hope due to the large amount of work directed at the issue.

"What is heartening to me is how collaborative the work is becoming," Rypel said, noting work between fisheries scientists and managers in Wisconsin, Minnesota, Michigan and Ontario.

"My hope is that our study will stimulate more thought into where and how we might make a difference for walleye management in the future. That way all our grandkids will still have a shot at catching walleye in Wisconsin, just like I did."